WHAT IF - you took the power supply out of a household LED lamp?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,881
That time again. Another failed household LED lamp. The gremlin I am just can't leave discovery alone. I've pealed the cover off the bulb, removed the LED PCB and heatsink. Pulled the power supply out of it. Does me no good because I'm fairly sure the failure is in the power supply for the LED's. Nevertheless I'm wondering what would happen if I took a GOOD working LED lamp apart and used the power supply for something else. No - I don't have a project in mind. But understanding how this works might spur a new idea. From what I can tell, it filters AC, rectifies it then oscillates it (likely at a very high frequency) then via board electronics, powers the LED's. And no, I don't know what voltage it's operating at. Nor do I know if it's a constant current setup.

My wonderings is what would happen if I removed the electronics from a good LED lamp and powered it from just - say - 24VAC instead of the designed 120VAC. If it's a constant current device - what current that might be? I have no idea. But I'm just wondering what such a power supply could be used for. Given that this is not one of those cheap chinesium capacitive voltage dropper circuits, if it's usable at a lower AC voltage, just what might I expect out of it?

So I thought I'd post this question and see what kind of responses I get. Please keep in mind I don't have any data on the PCB or the output of the PCB. CV or CC - I don't know. AND keep in mind I don't have a project for such an endeavor. Just that I get scrap stuff from time to time and wonder what I could repurpose it for. So I'd like to see what comes of this question. Thanks.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,881
Across the two leads (output) to the LED's is a 220µF electrolytic cap rated for 80V. So the output voltage must be less. The LED PCB itself has 22 LED's configured in two sets of 11 series LED's parallel to another series of 11 LED's.
 

Ian0

Joined Aug 7, 2020
5,504
Reassemble to lamp, connecting the LED array to the base. Then calculate the drive current by dividing the rated power by the voltage (which should be about 40V for 11 LEDs in series). Power it from an external constant current supply, and it will probably last for ever.
(But make sure you don't get it confused with a mains-operated lamp)
 

Jon Chandler

Joined Jun 12, 2008
251
Check out Big Clive on YouTube. He has taken apart and analyzed MANY LED bulbs. Current state-of-art bulbs use a bridge rectifier and an active-control constant current driver chip at line voltage. The current is set with a resistor or 2 resistors in parallel, and the chip throttles current when the temperature gets too high.
 

MisterBill2

Joined Jan 23, 2018
12,399
Note carefully which LED lead is positive, and then use your adjustable lab power supply to see what voltage the LEDs require. I have done this several times, and usually it is the power module for the LED that has failed. Only in the LED tubes used outsidehave I seen LED failures.
 

k1ng 1337

Joined Sep 11, 2020
584
Check out Big Clive on YouTube. He has taken apart and analyzed MANY LED bulbs. Current state-of-art bulbs use a bridge rectifier and an active-control constant current driver chip at line voltage. The current is set with a resistor or 2 resistors in parallel, and the chip throttles current when the temperature gets too high.
He does some interesting teardowns.

I seen a number of his LED videos, one in particular where he replaced a defective LED in a flood light and reduced output power by reconfiguring the resistors as you stated.

The bulbs he was dissecting seemed to be fairly universal and had useful components inside. On the other hand, some were made with the absolute minimum components and stressed them all..
 

MisterBill2

Joined Jan 23, 2018
12,399
Two recently dissected outdoor lights, same style but different sizes, have vastly different power supply modules. The one, with three LED assemblies in parallel, had a (failed) supply identified as "12 volts, 700 mA", and when I powered the leds off of my lab supply they drew 500 mA at about 10.2 volts, and were very bright. The smaller light, which has a single LED assembly, flickers, and does not deliver a useful amount of illumination now, has a supply marked "24 volts, 200mA".
My point being that the probability is that no two light assemblies have similar power supplies, and thus there is a need to check and know the voltage and current for the LED portion before substituting a different power supply.

In each case, the supply with the steel box housing, was partly covered with a tar-like material to prevent any repairs.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
6,881
Reassemble to lamp, connecting the LED array to the base. Then calculate the drive current by dividing the rated power by the voltage (which should be about 40V for 11 LEDs in series). Power it from an external constant current supply, and it will probably last for ever.
(But make sure you don't get it confused with a mains-operated lamp)
Please remember two things: First, this is a "What If" question; and second, the lamp in hand has failed. Reassembling it won't yield any useful information. But you said "which should be about 40V for 11 LEDs in series" which IS useful. Thanks for that.
Note carefully which LED lead is positive, and then use your adjustable lab power supply to see what voltage the LEDs require.
Unfortunately the variable PS I have is from a school electronics lab which consists of a ferroresonant transformer, auto-transformer, rectifier, choke and some caps to filter. There's also a high resistance 25 watt resistor across the DC output to drain the caps when the unit is shut down. There is no variable current, nor are there any readouts. A modern PS with CV & CC capabilities is something I would like to have - but don't.

I suppose from the stash of bulbs on hand I could pick one and do a more careful teardown and come away with an undamaged - working bulb PS and take some measurements. I'd start by measuring the active voltage at the LED's, then disassemble the LED's from the heatsink and hand solder some connecting wires to the other lamp that still has good (assumed) LED's and mount that to its own heat sink. Then interrupt the wiring to insert my meter set to measure current. But again, this is a "What If" question. I don't have a known purpose for this. I MIGHT if I knew what I had. I may further investigate, but that may happen in the coming weeks. Right now I'm working on construction of a cabinet for a client.

Thanks for the comments so far.
 

MisterBill2

Joined Jan 23, 2018
12,399
A ferro-resonant transformer with an isolated secondary, along with a rectifier, is a good start on a power supply. A fairly simple regulator controlled by an adjustable voltage regulator and you can have a decent supply,maybe zero to 24 volts at an amp or more. Ifyou can locate a used variac or powerstat variable voltage transformer and then add an isolation transformer you can have a variable, SAFE, but not regulated supply. Possibly even cheaper. And a multimeter is what you need to measure the voltage, add an ammeter shunt and you can read current as well. So there are cheap ways to have adjustable power.That was my situation until i was given the supply "that did not work", they could not get any output at all. What I discovered was that with the current limit set to zero and the over-voltage limit set to zero it was not possible to get any output. Setting the voltage and the current up a bit and it has worked well ever since. Some fixes are easier and simpler than others.
For LED experimenting you will sometimes need a higher voltage, up to possibly 100 volts for strings of LEDs. But the current will seldom be over 100 mA. I have not experimented with a light dimmer feeding a transformer/rectifier, but that is an other really simple kind of possibility.
 
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